U.S. patent number 6,201,576 [Application Number 09/105,783] was granted by the patent office on 2001-03-13 for apparatus and method for detecting an ntsc signal in an hdtv transmission signal.
This patent grant is currently assigned to Lucent Technologies Inc.. Invention is credited to Kalavai J. Raghunath, Marta M. Rambaud.
United States Patent |
6,201,576 |
Raghunath , et al. |
March 13, 2001 |
Apparatus and method for detecting an NTSC signal in an HDTV
transmission signal
Abstract
A system detects the presence of NTSC co-channel interference
and enables NTSC comb-filtering when the NTSC signal is detected.
The system comb-filters the baseband signal to generate a filtered
baseband signal, and accumulates the noise power of the baseband
and filtered baseband signals. The noise power of the baseband and
filtered baseband signals is compared by forming a difference
between the two noise powers, and the system detects the NTSC
signal when the difference exceeds a threshold T. The threshold T
is related to a product of a signal power of the baseband signal
and a minimum carrier to noise ratio for the ATSC system.
Inventors: |
Raghunath; Kalavai J. (Chatham,
NJ), Rambaud; Marta M. (Allentown, PA) |
Assignee: |
Lucent Technologies Inc.
(Murray Hill, NJ)
|
Family
ID: |
22307753 |
Appl.
No.: |
09/105,783 |
Filed: |
June 26, 1998 |
Current U.S.
Class: |
348/558;
348/E5.108; 348/21; 348/555; 348/556; 348/607; 348/554;
348/E5.077 |
Current CPC
Class: |
H04N
5/21 (20130101); H04N 21/42607 (20130101); H04N
5/4401 (20130101); H04N 21/426 (20130101) |
Current International
Class: |
H04N
5/21 (20060101); H04N 5/44 (20060101); H04N
007/00 () |
Field of
Search: |
;348/21,558,535,554,555,556,607,618,622,665,667,454,92,130,142
;375/346,350,285 ;455/63,296,306 ;382/143,160 ;355/101,106,107 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Luu; Matthew
Assistant Examiner: Sajous; Wesner
Attorney, Agent or Firm: Hughes; Ian M. Mendelsohn;
Steve
Claims
What is claimed is:
1. An apparatus for detecting an NTSC signal in a baseband signal
comprising:
at least one comb-filter, a portion of the baseband signal applied
to the comb-filter to generate a filtered baseband signal
portion;
a first noise power accumulator for accumulating a noise power of
the baseband signal portion;
a second noise power accumulator for accumulating a noise power of
the filtered baseband signal portion;
a difference generator which forms a difference between the noise
powers of the baseband signal and filtered baseband signal
portions; and
a comparator for detecting the NTSC signal when the difference
exceeds a threshold T,
wherein the threshold T during detection is related to a product of
a measured signal power of the baseband signal and an inverted
minimum carrier to noise ratio of the baseband signal.
2. The apparatus as recited in claim 1, wherein each of the first
and second noise power accumulators comprise a squaring circuit for
squaring the baseband or filtered baseband signal portion, and an
integrator for integrating the squared signal from the
corresponding squaring circuit to provide the accumulated noise
power.
3. The apparatus as recited in claim 2, wherein the baseband signal
includes a data field sync signal and the apparatus includes at
least two comb filters, the first comb-filter filtering the portion
of the baseband signal corresponding to the data field sync signal
and the second comb-filter filtering a data field sync reference
pattern, the apparatus further comprising:
first and second subtractors, the first subtractor forming a
difference of the data field sync signal and the data field sync
reference pattern, the difference provided to the first noise power
accumulator, and the second subtractor forming a difference of the
filtered data field sync signal and the filtered data field sync
reference pattern, the difference provided to the second noise
power accumulator.
4. The apparatus as recited in claim 3, wherein each comb-filter
comprises a subtractor and a delay, the delay providing a delayed
the baseband signal, and the subtractor forming a difference of the
baseband and delayed baseband signals to provide the filtered
baseband signal.
5. The apparatus as recited in claim 4, wherein the apparatus is
included in a video receiver having a mux, the baseband signal is
an encoded video signal having a data signal and the data field
sync signal, the first comb-filter further filters the data signal,
and the comparator provides a control signal to the mux to select
the filtered data signal and filtered data field sync signal when
the NTSC signal is detected and to select the baseband signal when
the NTSC signal is not detected.
6. A method of detecting an NTSC signal in a baseband signal
comprising the steps of:
a) comb-filtering a portion of the baseband signal to generate a
filtered baseband signal;
b) generating an expected value related to a noise power of the
baseband signal portion;
c) generating an expected value related to the noise power of the
filtered baseband signal portion;
d) forming a difference between the expected values of the baseband
signal and the filtered baseband signal portions; and
e) detecting the NTSC signal when the difference exceeds a
threshold T, wherein T during detection is related to a product of
a measured signal power of the baseband signal and an inverted
minimum carrier to noise ratio of the baseband signal.
7. The method of detecting an NTSC signal as recited in claim 6,
wherein the baseband signal includes a data field sync signal and
the comb-filtering step a) further includes the steps of:
a1A) comb-filtering the portion of the baseband signal
corresponding to the data field sync signal;
a1B) comb-filtering a data field sync reference pattern
a1C) removing the data field sync signal from the portion of the
baseband signal based on the data field sync reference pattern;
and
a1C) removing the filtered data field sync signal from the filtered
portion of the baseband signal based on the filtered data field
sync reference pattern.
8. The method of detecting an NTSC signal as recited in claim 7,
wherein the generating step b) generates the expected value related
to the noise power of the portion of baseband signal corresponding
to the data field sync signal and the generating step c) generates
the expected value related to the noise power of the filtered
portion of baseband signal corresponding to the filtered data field
sync signal.
9. The method of detecting an NTSC signal as recited in claim 8,
wherein the baseband signal having a data signal and the data field
sync signal is an encoded video signal, the comb-filtering step a)
further comprises step a2) of comb-filtering a remaining portion of
the baseband signal corresponding to the data signal, and the
method further comprises the step of providing the filtered
baseband signal when the NTSC signal is detected by the detecting
step e) and providing the baseband signal when the NTSC signal is
not detected by the detecting step e).
10. An apparatus for detecting an NTSC signal in a baseband signal
comprising:
comb-filtering means for comb-filtering a portion of the baseband
signal to generate a filtered baseband signal portion;
first accumulating means for accumulating a noise power of the
baseband signal portion;
second accumulating means for accumulating a noise power of the
filtered baseband signal portion;
difference means for forming a difference between the noise powers
of the baseband and filtered baseband signal portions; and
comparing means for comparing the difference with a threshold T,
the NTSC signal detected when the difference exceeds the threshold
T, wherein T during detection is related to a product of a measured
signal power of the baseband signal and an inverted minimum carrier
to noise ratio.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to digitally encoded television
transmission systems, and, more particularly, to systems detecting
NTSC signals.
2. Description of the Related Art
In the United States, the Advanced Television Systems Committee
(ATSC) has proposed a digital television standard for High
Definition Television (HDTV) transmission systems. A typical
transmitter 100 and receiver 120 of an ATSC transmission system is
shown in FIG. 1. The transmitter 100 comprises a video encoder 102
for compressing digital video signals, an encoder & trellis
coder 104 for Reed-Solomon coding and trellis coding the signal
from video encoder, a precoder 106 for preceding the signal output
from encoder & trellis coder 104. Precoding by precoder 106
combines selected symbols of the data stream in a manner that is
reversed by an NTSC filter in the receiver 120, thereby canceling
NTSC interference as described subsequently. The transmitter 100
also comprises a modulator & SAW filter 108 for forming the
signal output from precoder 106 into a form of vestigial side band
within 6 MHz, and a radio frequency (RF) transmitter 110 for
transmitting the signal from modulator & SAW filter 108 through
an RF channel 112.
The receiver 120 comprises a radio frequency (RF) tuner 121
including an intermediate frequency (IF) surface acoustic wave
(SAW) filter for selecting a RF channel and providing an IF signal.
The IF signal is provided to a demodulator 122 to provide a
baseband signal, known as an I-channel signal, and timing recovery
circuit 123 recovers data clock, synchronization and timing clock
signals from the I-channel signal containing composite symbols for
data and timing. The demodulator 122 also may include a synchronous
detector and analog-to digital converter (not shown) which provides
the I-channel signal as digital samples. An NTSC detector and
rejection filter 124, which may be a comb-filter and controller,
detects and cancels NTSC co-channel interference in the baseband
I-channel signal. A channel equalizer 125 compensates for
distortion of the I-channel signal by the RF channel 110 and
distortion of the comb-filter, if used, of NTSC detection and
rejection filter 124. The I-channel data symbols of the compensated
I-channel signal are then applied to a bit de-interleaver (not
shown) and error correction and trellis decoding circuitry 126
which performs Reed-Solomon decoding and trellis decoding of the
I-channel data symbols to form a decoded bit stream. The decoded
bit stream from the error correction and trellis decoding circuitry
126 is then reformatted to a digital data stream by deformatter
128. Deformatter 128 reformats the decoded bit stream since the
original digital data stream of an encoder is formatted so as to
appear as a random bit stream. The reformatted digital data stream
is then decoded by video decoder 130 to provide video signals.
NTSC interference rejection is based on the frequency location of
the NTSC co-channel interfering components with respect to
transmitted HDTV signals, which relationships are illustrated in
FIGS. 2A-2C. FIG. 2A illustrates a RF spectrum of a HDTV signal as
transmitted. FIG. 2B illustrates a RF spectrum of an NTSC signal
that may cause co-channel interference. FIG. 2C illustrates
frequency characteristics of a comb filter as typically used to
remove NTCS co-channel interference.
As shown in FIG. 2B, the NTSC signal includes picture carrier,
color sub-carrier and audio carrier signals. The comb filter
frequency characteristics have null points spaced 896.85 kHz apart
which null points are around the frequencies of the picture
carrier, color sub-carrier and audio carrier signals. Passing the
NTSC signal through a comb filter having such characteristics
removes these carrier signals. FIG. 3 is a block diagram of a
conventional NTSC comb filter 300. As shown in FIG. 3, the filter
300 may be a single tap, feed forward filter and comprises a delay
301 and subtractor 302. Delay 301 provides a delayed I-channel
signal, to subtractor 302, and delay 301 typically delays the
I-channel symbols by 12 symbols. Since the comb-filter forms a
difference of a symbol and a delayed symbol, the precoder 106 of
the transmitter anticipates the comb-filtering and adjusts each
symbol accordingly.
The conventional NTSC comb-filter 300 as shown in FIG. 3, while
providing rejection of steady state signals at null frequencies has
a finite response of, for example, 12 symbols. In addition, while
the comb filter reduces NTSC co-channel interference, the data is
also modified. As a result of the single tap filter forming a
difference of two full gain paths, the comb filter decreases
signal-to-noise ratio, degrading white noise performance by 3 dB.
Consequently, the ATSC transmission system only comb-filters when
necessary. Therefore, an ATSC receiver 120 includes an NTSC
detector that only enables NTSC filtering and equalizes the
baseband signal when the presence of the NTSC signal is
detected.
These NTSC detectors of the prior art typically monitor the signal
energies of the un-filtered and filtered baseband signals, and only
enable the NTSC comb filter when a SNR drop of greater than 3dB
occurs. When an NTSC signal is not present in the baseband signal,
filtering doubles the noise power, or reduces SNR by 3 dB, in the
filtered signal. A minimum energy detector, therefore, may be used
to compare interference noise power, u.sup.2, of the baseband
signal with the interference noise power, f.sup.2, of the filtered
baseband signal. If u.sup.2 is greater than f.sup.2 /2, then the
NTSC signal is present and filtering is enabled.
Since the I-channel signal includes both a data component (data
symbols) and timing component (data field sync signal), an NTSC
detector of the NTSC detection and rejection filter 104 of FIG. 1
typically measures a signal-to-interference plus channel noise
ratio of the data field sync signal path. This measurement is
typically performed by creating and comparing two error signals.
The first error signal is created by comparing the received signal
with a stored reference of the data field sync signal, and the
second error signal is created by comparing the comb-filtered data
field sync signal with a comb-filtered version of the data field
reference signal. Consequently, the NTSC detector includes a second
NTSC filter which comb filters the extracted data field sync
signal.
SUMMARY OF THE INVENTION
The present invention relates to an apparatus and method of
detecting an NTSC signal in a baseband signal. First, the baseband
signal is comb-filtered to generate a filtered baseband signal.
Then, an expected value related to the noise power of the baseband
signal and an expected value related to the noise power of the
filtered baseband signal are generated. A difference is formed
between the expected values of the baseband and filtered baseband
signals; and the NTSC signal is detected when the difference
exceeds a threshold T. Threshold T is related to a product of a
signal power of the baseband signal and a minimum carrier to noise
ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects, features, and advantages of the present invention
will become more fully apparent from the following detailed
description, the appended claims, and the accompanying drawings in
which:
FIG. 1 shows a block diagram of a typical transmitter and receiver
of an ATSC transmission system;
FIG. 2A illustrates a RF spectrum of a HDTV signal as
transmitted;
FIG. 2B illustrates a RF spectrum of an NTSC signal that may cause
co-channel interference;
FIG. 2C illustrates frequency characteristics of a comb filter as
typically used to remove NTCS co-channel interference;
FIG. 3 is a block diagram of a conventional NTSC comb filter;
and
FIG. 4 is a block diagram of an NTSC comb filter and NTSC signal
detector in accordance with an exemplary embodiment of the present
invention as employed by an ATSC system as shown in FIG. 1.
DETAILED DESCRIPTION
In accordance with the present invention, an NTSC signal is
detected in a baseband signal by comb-filtering the baseband signal
to generate a filtered baseband signal, accumulating the noise
power of the baseband and filtered baseband signals, forming a
difference between the noise powers of the baseband and filtered
baseband signals; and detecting the NTSC signal when the difference
exceeds a threshold T, the threshold T related to a product of a
signal power of the baseband signal and a minimum carrier to noise
ratio for the ATSC system.
FIG. 4 is a block diagram of an NTSC comb filter and an
interference detector in accordance with the present invention as
may be employed in a NTSC detection and rejection filter 104 of
FIG. 1 As shown in FIG. 4, the NTSC comb filter and interference
detector 400 in accordance with the present invention comprises an
NTSC filter 404 made up of a delay 405 and a subtractor 406 for
removing an NTSC interference component from the received I-channel
signal, and an NTSC filter 414 made up of a delay 415 and
subtractor 416 for removing an NTSC component from a data field
sync reference pattern.
The detector 400 also comprises first and second signal noise power
accumulators 430 and 440. The first signal noise power accumulator
430 comprises subtractor 402 for obtaining the difference between a
data field sync signal of a received I-channel signal and a
reference pattern data field sync, a squaring circuit 420 for
squaring an absolute value of the signal output from subtractor
402, and an integrator 421 for integrating the signal from squaring
circuit 420 for a predetermined time to form a first error signal
u.sup.2.
The second signal noise power accumulator 440 comprises subtractor
403 for obtaining the difference between the comb-filtered data
field sync signal of the I-channel from NTSC filter 404 and the
comb-filtered reference data field sync signal from NTSC filter
414, a squaring circuit 422 for providing a squared, absolute value
of the signal from subtractor 403, and an integrator 423 for
integrating the signal from squaring circuit 422 for a
predetermined time to form a second error signal f.sup.2.
The detector 400 further comprises a minimum energy detector 425
for comparing the signals u.sup.2 and f.sup.2 from integrators 421
and 423, determines the lowest noise-energy signal, and forms a
control signal for controlling multiplexer 426 to select one of the
signal passing through NTSC filter 404 and the received I-channel
signal. The minimum energy detector 425 selects a lowest
noise-energy signal between the two signals based upon equation
(1):
where T is a threshold value and determined as described
subsequently. As shown in FIG. 4, the threshold value T may be
provided by a T calculation process 427 base upon a measured signal
power s.sup.2.
If the signal u.sup.2 from integrator 421 meets the conditions of
lower noise-energy, minimum energy detector 425 determines that
there is no NTSC co-channel interference components to the received
I-channel signal, and so provides the control signal so that
multiplexer 426 selects the received and unfiltered I-channel
signal. If the signal f.sup.2 from integrator 423 has lower
noise-energy, minimum energy detector 425 determines that NTSC
co-channel interference components are present and provides the
control signal so that multiplexer 426 selects the filtered
I-channel signal from NTSC filter 404.
For the detector 400 as shown in FIG. 4, the unfiltered and
filtered noise power in the data field sync signal is accumulated
as u.sup.2 and f.sup.2, respectively. The squaring circuits 420 and
422 and integrators 421 and 423 are desirably enabled during a
period when data field sync signal symbols are present. A data
field sync reference pattern is present in the receiver itself, and
timing information is recovered from the data clock recovery
portion 123 of the receiver of FIG. 1. Out of the received
I-channel signal, the NTSC component of the data field sync signal,
if present, is canceled through NTSC filter 404. As is known in the
art, an NTSC filter also distorts the data field sync signal to
some extent. Consequently, the signal of the data field sync
reference pattern is also passed through an NTSC filter 414.
Consequently, minimum energy detector 425 compares noise power of
the data field sync signal and data field sync reference pattern
passing NTSC filters 404 and 414, respectively, with the data field
sync signal and data field sync reference pattern of the I-channel
signal, thereby outputting a control signal for controlling
multiplexer 426 according to the comparison result.
The process for determining the threshold value T of equation (1)
is now described. The I-channel signal, i(t), having the data field
sync signal components removed may be represented as in equation
(2a), and the filtered I-channel signal, i'(t), having the data
field sync signal components removed may be represented as in
equation (2b): ##EQU1##
In equations (2a) and (2b), n.sub.i (t) is the NTSC interference
noise, n(t) is the channel noise added from the communication
channel and 2n(t) is the doubled channel noise by comb-filtering.
Squaring i(t) and taking the expected value, then the noise power
of the unfiltered I-channel signal, u.sup.2, and the noise power of
the filtered I-channel signal, f.sup.2, is given in equations (3a)
and (3b), respectively:
where N.sup.2 is the noise power of the NTSC interference noise,
and n.sup.2 is the channel noise power.
Rearranging equations 3a and 3b gives N.sup.2 as in equation
(4):
However, if NTSC noise is present, then the signal to noise ratio
of the unfiltered signal must be less than the signal to noise
ratio of the filtered signal, or equation (3a) is greater than
equation (3b), which yields equation (5):
In accordance with the present invention, the threshold level, T,
of equation (1) is calculated employing the a-priori information
that the carrier-to-noise ratio (C/N) threshold for the ATSC system
is a predetermined level. The C/N threshold expressed as 10.sup.Y,
given as C/N=s.sup.2 /n.sub.max.sup.2 where n.sub.max.sup.2 is the
maximum channel noise-power, and does not include noise from
potential NTSC co-channel interference since NTSC rejection
filtering at the filter is assumed. The C/N threshold for the ATSC
system may be, for example, 14.9 dB for terrestrial mode or 28.3 dB
for cable mode. Since n.sub.max.sup.2 must desirably be greater
than n.sup.2 for the ATSC system, equation (5) results:
Substituting equations (4) and (5) into equation (6) yields an
equation (7) giving the threshold level T:
In accordance with the present invention, the threshold level T for
the minimum energy detector 425 of FIG. 4 compares the signals from
integrators 421 and 423 with s.sup.2 10.sup.-y, and the values for
s.sup.2 and 10.sup.-y are known to the receiver 120 (FIG. 1). As
described previously, 10.sup.-y is known from the ATSC C/N
requirement, and s.sup.2 is determined in the receiver 120. For
example, some demodulator circuits for demodulator 122 may have a
variable gain amplifier at a front end of receiver 120 which sets
signal power, letting noise power, n.sup.2, vary as a function of
gain for the receiver 120. In this case, s.sup.2 is a fixed value,
and so T is a fixed value. For an alternate case, the signal to
noise ratio varies, but the demodulator may measure signal power
s.sup.2. In this case, s.sup.2 is a variable value, and so T is a
variable value, and minimum energy detector 425 may adaptively vary
the threshold value T during each comparison of noise powers
u.sup.2 and f.sup.2 from integrators 421 and 423.
While the exemplary embodiments of the present invention have been
described with respect to processes of circuits, the present
invention is not so limited. As would be apparent to one skilled in
the art, various functions of circuit elements may also be
implemented in the digital domain as processing steps in a software
program. Such software may be employed in, for example, a digital
signal processor, micro-controller or general purpose computer.
It will be further understood that various changes in the details,
materials, and arrangements of the parts which have been described
and illustrated in order to explain the nature of this invention
may be made by those skilled in the art without departing from the
principle and scope of the invention as expressed in the following
claims.
* * * * *